An other implementation of built-in generator `itertools.combinations` in pure Python. This function is faster than the official demostration in some cases.

combinations.py

def combinations(iterable, r, start=0):
    '''
    A generator that yields the combinations of the iterable object.

    This generator has the same interface as the built-in generator
    `itertools.combinations`. This function is faster than the official
    demostration when the r is closed to the half of length of the input
    iterable object.
    '''
    pool = tuple(iterable)
    l = len(pool)
    if r == 0 or l - start < r:
        return
    for i in range(start, l):
        if r == 1:
            yield (pool[i], )
        else:
            for res in combinations(pool, r - 1, i + 1):
                yield (pool[i], ) + res


def offical_combinations(iterable, r):
    '''
    link: https://docs.python.org/zh-cn/3/library/itertools.html#itertools.combinations
    '''
    # combinations('ABCD', 2) --> AB AC AD BC BD CD
    # combinations(range(4), 3) --> 012 013 023 123
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
        return
    indices = list(range(r))
    yield tuple(pool[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else:
            return
        indices[i] += 1
        for j in range(i+1, r):
            indices[j] = indices[j-1] + 1
        yield tuple(pool[i] for i in indices)

task1 = \
'''
for c in combinations({}, 7):
    pass
'''

task2 = \
'''
for c in offical_combinations({}, 7):
    pass
'''

task3 = \
'''
for c in itertools.combinations({}, 7):
    pass
'''

a7 = [1, 2, 3, 4, 5, 6, 7]
a10 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
a13 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

if __name__ == '__main__':
    import timeit
    print(
        timeit.timeit(task1.format('a7'), 'from combinations import combinations, a7', number = 1000),
        timeit.timeit(task2.format('a7'), 'from combinations import offical_combinations, a7', number = 1000),
        timeit.timeit(task3.format('a7'), 'from combinations import a7; import itertools', number = 1000),
    )
    print()
    print(
        timeit.timeit(task1.format('a10'), 'from combinations import combinations, a10', number = 1000),
        timeit.timeit(task2.format('a10'), 'from combinations import offical_combinations, a10', number = 1000),
        timeit.timeit(task3.format('a10'), 'from combinations import a10; import itertools', number = 1000),
    )
    print()
    print(
        timeit.timeit(task1.format('a13'), 'from combinations import combinations, a13', number = 1000),
        timeit.timeit(task2.format('a13'), 'from combinations import offical_combinations, a13', number = 1000),
        timeit.timeit(task3.format('a13'), 'from combinations import a13; import itertools', number = 1000),
    )